It is not clear why some people develop posttraumatic stress disorder (PTSD) in response to a traumatic event. DNA methylation, an epigenetic mark that associates with trauma and other environmental exposures, predicts PTSD in multiple studies, and methylation of some genes may be informative for early prediction and treatment of PTSD. The purpose of this study is to facilitate an epigenome-wide association study (EWAS) of PTSD in participating cohorts in Psychiatric Genomics Consortium (PGC) PTSD workgroup. In this study, we will leverage thousands of cross-sectional and longitudinal samples of PTSD cases and trauma-exposed controls with methylome data collected using the same genome-wide array. The meta-analyses of these samples will be allow for identification of DNA methylation patterns that predict PTSD in diverse subjects as well as those that are sex-specific or specific to type of trauma (child vs. adult or combat vs. civilian). DNA methylation patterns that are most predictive of PTSD will be replicated in an independent cohort of cases and controls. The results of this study provide insight into the biologic pathways underlying the development and progression of PTSD, will complement ongoing efforts to identify therapeutic targets for PTSD, and will inform prospective studies of PTSD and trauma exposure that are underway.